Partial Distribution Functions Research Articles

Selective laser sintering of amorphous nanoparticles: Molecular dynamics simulations

The paper investigates the process of liquid-phase sintering of amorphous iron-based nanoparticles by the method of molecular dynamics simulations. The classical molecular dynamics package LAMMPS was used for modeling. Visual analysis of the atomic configurations of nanoparticles during their rapid cooling revealed the self-purification effect of the particles. Partial pair correlation functions and coordination number distribution functions were used to analyze the atomic structure of nanoparticles after sintering. As a result of the analysis of the main structural parameters, which were obtained using the specified functions, differences in the atomic composition and structure of the volume and surface of nanoparticles were established.

Partial EEDF analysis and electron diagnostics of atmospheric-pressure argon and argon–helium DBD plasma

Partial arbitrary electron energy distribution function (EEDF) results for cold atmospheric-pressure plasma are reported. The EEDF is obtained using the visible bremsstrahlung inversion (VBI) method. This machine learning method requires only optical emission spectroscopy (OES) measurement and a momentum transfer cross section to determine a partial EEDF. Numerical EEDF of a pure-argon dielectric barrier discharge dataset with changing peak-to-peak voltage and a helium–argon discharge with changing mixture ratio are reported. Resemblance between the numerical EEDF and a two-temperature Maxwell distribution is observed and a simplified three-point numerical EEDF is obtained. The electron temperature and relative electron number density for the bulk and high-energy electron populations are measured. The bulk electron temperature was consistently 0.3 eV. For pure argon, the high-energy electron temperature decreased exponentially from 3 to 2.2 eV with increasing peak-to-peak voltage from 3.6 to 6.3 kV. For the helium–argon dataset, the high-energy electron temperature decreased linearly from 4.2 to 2.2 eV with increasing argon fraction 25%–100%. From an OES measurement, the arbitrary EEDF can be observed by utilization of the VBI method. Based on this numerical EEDF, appropriate assumptions can be applied to simplify the quantification of electron diagnostics.

Investigation on the local structure and properties of molten Li2CO3-K2CO3 binary salts by machine learning potentials

In recent years, (LinK1-n)2CO3 binary melts have been investigated as the promising electrolyte matrixes of the fuel cell. Many researchers moved to the investigation of local structure and properties of the melts. In this paper, machine learning interatomic potentials were developed based on first-principle datasets in order to research the local structure and properties of molten (LinK1-n)2CO3 (n = 0.4,0.5,0.6) binary salts. The trained machine learning potentials, which enable similar accuracy relative to DFT and high efficiency, can yield precise descriptions of microstructure and macro properties of Li2CO3-K2CO3 binary melts. The information of the microstructure evolution was analyzed through partial radial distribution functions, coordination numbers and bond angle distribution functions. It is observed that lithium cation exhibits more stable coordination and destroys the C-O ion pairs. Further, comparing the inter-ionic partial radius distribution function diagrams of binary melts and those of pure Li2CO3 and K2CO3 melts, the changing regularities of interatomic strength and arrangement were obtained. Macro properties including density, self-diffusion coefficients, thermal conductivity and viscosity at target temperature range were calculated by the potentials and MD simulations. The temperature effect and concentration effect in properties were explored. This work exhibits a thorough understanding of the local structure and properties of Li2CO3-K2CO3 melts and reveals the accuracy of machine learning potentials on lithium-potassium melts for the first time.

Structural features and rare earth ion clustering behavior in lanthanum phosphate and aluminophosphate glasses from molecular dynamics simulations

This paper reports detailed short and medium range structural features of lanthanum oxide containing alkaline phosphate and aluminophosphate glasses revealed from molecular dynamics simulations. Two glass series xLa2O3-(100-x)NaPO2 (x = 0–7.5) and xLa2O3-yAl2O3-(100-x-y) NaPO3 (0 ≤ x≤ 5 and 5 ≤ y ≤ 15) were studied and the cation local environments as a function of compositions in terms of partial distribution functions, bond angle distributions and oxygen coordination number investigated. It was found that La-O has a bond distance of ∼x223C2.44 Å and average coordination number 6.5-7, consistent with experimental findings based on neutron and X-ray diffractions. Medium range structure feactures such as Qn distribution, network connectivity and polyhedral linkages were studied. The clustering behavior of lanthanum ions in the glasses were investigated in detail and it was found that La3+ ions are mainly surrounded by PO4 units in the second coordination shell that prevented La3+ ion from clustering. As a result, the addition of alumina does not significantly change the clustering behavior.

Structural and thermodynamic properties for the BaMn(Fe=V)F7 Fluoride glass system by using the Hybrid Reverse Monte Carlo simulation

The Hybrid Reverse Monte Carlo (HRMC) simulation has been widely used as a very useful method for displaying the pair partial distribution functions (PDFs) g(r) eliminating as soon as possible the artificial satellite peaks appear by the RMC simulation. The HRMC is an extension of the RMC algorithm, which introduces an energy penalty term (potential) in the acceptance criteria.The glass retains the structure presented by the liquid at the glass transition temperature Tg, and the thermodynamic properties are influenced by these structural modifications. We are interested in this study to apply the structural parameters g(r), obtained from HRMC simulation, to determine some structural and thermodynamic properties for the BaMn(Fe=V)F7 Fluoride glass.The calculated structural properties such as the running coordination number n(r) were in good agreement with coordination constraint. We suggest also that the structural parameters g(r) is a good tool to determine the thermodynamic properties as the energy of the system.

Modification of the singlet equation for a molecular system of solid spheres near a solid surface in the Percus-Yevick approximation

An analysis of theoretical methods for studying the molecular structure of liquids bordering a solid surface was carried out. It was established that the currently existing nonlinear integral equations for partial distribution functions do not have an analytical solution for spatially inhomogeneous liquids. A linear integral equation having an analytical solution for a system of solid spheres near a solid surface was proposed.

Short- and Intermediate-Range Structure and Dynamics of Fe-Ni-C Liquid Under Compression

Properties of liquid Fe alloys under high-pressure conditions are crucial for understanding the composition, thermal state, and dynamics of Earth’s core. Experiments on such liquids, however, are often performed under pressures far below those of the outer core, necessitating long extrapolations of experimental results to core conditions. Such estimates can be complicated by light elements possibly forming pressure-dependent molecular clusters that can significantly affect the physical properties of liquids as core conditions are approached. First-principles molecular dynamics simulations were employed to compute the properties of an Fe-Ni-C liquid with a composition of Fe3.7Ni0.37C at 1673 K and pressures from 0 to 67 GPa to benchmark computational methods on pressure effects on the structure and properties of the liquid relative to low pressure experimental results. The short-range structure is manifested by the coordination number (CN) of Fe/Ni-Fe/Ni being around 12, indicative of a nearly close-packed structure in the pressure range, and the CN of C-Fe/Ni gradual increasing from 6.5 to 8.5, indicative of an approximately octahedral to cubic transition as pressure increases. The Fe/Ni-Fe/Ni bond distance, however, is found to be 10 times more compressible than the C-Fe/Ni distance. The intermediate-range structure of Fe/Ni-Fe/Ni and C-Fe/Ni subsystems, described by a partial configurationally-decomposed distribution function, undergoes substantial changes, characterized by a significant increase of the number of polyhedra that share 3 atoms with each other. Such dense configurations are related to an increased bulk modulus, decreased diffusion coefficient, decreased activation volume for diffusion, and increased shear viscosity. Reproducing the experimental observations at low pressures provides important support for modeling the liquid under the conditions relevant to the outer core.

Influence of mechanical tensile and compression tests under high strain rate on structural properties of copper monatomic metallic glass

This paper aims to investigate the structural properties of Cu-monatomic metallic glass, and to elucidate the effect of mechanical tensile and compression tests with high strain rate on these properties. This work is performed by using molecular dynamics simulations combined with embedded atom method. The structure parameters such as radial distribution function, Voronoi Tessellation and Common neighbor analysis have been used to characterize and investigate the local structure in the Cu-monatomic metallic glass. Our numerical results show that, the most dominant structures are the distorted icosahedra ⟨0,1,10,2⟩ and ⟨0,2,8,4⟩ for this specific system. These two structures are more higher than the full icosahedra motif ⟨0,0,12,0⟩ which presents, generally, the high percentage for multi-component and for some monatomic metallic glasses. In addition, we have found that the second peak splitting in the partial distribution function of Cu metallic glass is caused essentially by the icosahedra-like polyhedron and not only by the full icosahedra. On the other hand, we have shown that the mechanical tests strongly influence these local structures. In fact, tensile and compression tests lead to reduce the importance of the short range order by decreasing the icosahedra and icosahedra-like fractions. However, these mechanical effects do not alter significantly ⟨0,3,6,4⟩ clusters, which maintain almost the same percentage during the mechanical testing. Moreover, comparison between compression and tensile tests with regard to the mechanical behavior and how it can be altered by the strain rate is done. Implications of these results are briefly discussed.

Annealing-assisted high-pressure torsion in Zr55Cu30Al10Ni5 metallic glass

For bulk metallic glasses (BMGs) structural anisotropy has been studied for many years, but usually for small creep strains, rather than for large strains associated with the viscoplastic flow. In this study, the structural modifications in the short-to-medium range order of Zr55Cu30Al10Ni5 BMG is deliberated for the first time upon simultaneous high-pressure torsion (HPT) and in-situ annealing around the glass transition temperature (Tg). The changes are evaluated in terms of hardness by microindentation, thermal properties by differential thermal calorimetry, chemistry by energy dispersive X-ray, and structural properties by synchrotron X-ray diffraction and transmission electron microscopy. A remarkable increase of the glass transition temperature and the absence of the shoulder of the crystallization peak along with remarkable softening and shear thinning reveals that even at Tg the material gains flow characteristics because of the extreme pressure applied. A clear shift towards smaller wave vector Q in the peak positions of the partial distribution function G(r), and reconfiguration of the bonding between Zr−Zr and Zr−Cu atomic pairs extracted from the deconvolution of the radial distribution function RDF(r) are observed with changing the processing temperature. The findings give insight into the atomic and nano-scale modifications in BMGs due to high-pressure torsion at different temperatures which would be essential to tune the intrinsic properties of BMGs to attain the desirable hardness and thermal stability for a specific application.

Microdomain atomic structure of Zr50Pd40Al10 metallic glasses and its formation mechanism

Zr-based Zr50Pd40Al10 metallic glasses has not only crystalline phases of about 5 nm in diameter but also amorphous phases. In this work, the radial distribution functions (RDFs) of amorphous structure of Zr50Pd40Al10 metallic glasses were firstly measured by electron diffraction, and then Reverse Monte Carlo (RMC) optimization accompanied by density functional theory (DFT) calculations. The amorphous structure has not only short-range order but also good medium-range order. In the RDFs of its amorphous structure, the first and the second peaks are located at 2.96 Å and 4.79 Å, respectively. Partial radical distribution functions (PRDFs) show that the contributions of the first and the second nearest-neighbor distances of various atom pairs to the G(r) peak values, and the first nearest-neighbor distances of Pd–Zr and Zr–Zr atom pairs are the sources of main G(r) peak values between 2 Å and 6 Å. The competition mechanism for generating the Pd25Zr55Al20 amorphous phase and the intermetallic crystalline phase Pd11Zr9 is associated with the differences of atomic radius, the proportion, and the melting point of different atoms, as well as the heat of mixing between atoms, leading to an equilibrium state of the two phases. Accordingly, a composite system with intertwined nanocrystals and amorphous phases is in turn formed, and improves the stability of the material.

Comparing short–range and medium–range ordering in Cu[sbnd]Zr and Ni[sbnd]Zr metallic glasses – Correlation between structure and glass form ability

According to recent studies, CuZr and NiZr binary alloying systems are very similar based on known glass-forming ability criteria, however, they exhibit significant difference in glass-forming ability in practice. In this work, local atomic structures of CuZr and NiZr metallic glasses are studied by molecular dynamics simulation to explain the source of mentioned difference. The total and partial distribution functions, coordination number and voronoi analysis are utilized to characterize the local atomic structures around Zr, Ni and Cu atoms. It was found that the local environment around Zr atoms is almost similar in both systems. The difference in the atomic structure in these systems mainly arises from topologies of polyhedra around Cu and Ni atoms. In particular, full icosahedron is one of the most popular local structure in CuZr system, while its population is significantly smaller in NiZr system with the same composition. Additionally, networks formed by full icosahedra connections, medium range order, are more robust in CuZr system. Higher percentage of topologically ordered and compact structures and their connectivity could explain why bulk metallic glass formation is possible in CuZr system but not in NiZr system.

Use of an improved high-throughput absolute abundance quantification method to characterize soil bacterial community and dynamics

High-throughput sequencing has dramatically expanded our understanding of bacterial communities based on the information of the species types and their relative abundances. Recently, researchers have also become aware of a deficiency in not considering the absolute abundance in this technique. Combining two or more different methods has typically been used to achieve absolute quantification of microbial communities. However, making a combination of different methods not only is time-consuming but also involves potential uncertainty due to variations in the experimental conditions. To simplify the experimental procedure and improve the high-throughput absolute abundance quantification (HAAQ) of a soil bacterial community, we propose an HAAQ method that uses an internal standard strain (ISS) HAAQ-GFP to simultaneously obtain both the relative and absolute abundances in the soil bacterial community. The results showed that a soil bacterial community and its dynamics can be better characterized by the HAAQ method when the optimal concentrations of ISS HAAQ-GFP (105 to 107cellsg−1) were used, and a 16S rRNA gene copy number adjustment was applied. Based on the HAAQ method, we first found that soil bacterial absolute abundances at the genus level fitted well to the partial log-normal distribution function, and most genera concentrations were in the range of 103.5 to 106.5cellsg−1 in the test soils. Our case studies also indicated that more comprehensive descriptions of soil bacterial communities and their dynamics can be achieved by both the relative and absolute abundances than by the relative abundance alone. The improved HAAQ method can be potentially applied to other microbial ecological studies and to stimulating the development of quantitative bacterial ecology studies.

First principle study on atomic structure of La65X35(X=Ni, Al) metallic glasses

The atomic-level structure of metallic glasses (MGs) is one of the most fundamental and challenging topics in condensed matter physics. Unlike crystalline metals or alloys, the MGs are lacking in a well-defined description of structure order, which is a major obstruction for relating its structure to physical properties. Obviously, it is vitally important to have an in-depth understanding of the atomic packing scheme in MGs. Due to the limitations of experimental characterization methods, it is hard to obtain the atomic packing scheme of MGs in experiment. Computational simulation on an atomic scale has become an important method of characterizing the atomic structure of MGs. The La-based LaNiAl glass forming system is well-known for its good glass-forming ability, distinctive relaxation peak that is well separated from relaxation, and liquid-liquid transition at a temperature around 1000 K. Many efforts have been made to investigate these novel properties. However, the atomic structure of this system is rarely studied. In this paper, the atomic structure evolution from liquids to glass states in La-based binary MGs La65Ni35 and La65Al35 are studied via ab initio molecular dynamics based on the density functional theory. The local structures are systematically analyzed by the radical distribution function, partial radical distribution function (PRDF), Voronoi tessellation method, and bond-type method in Honeycutt-Andersen. The results indicate that the PRDF of NiNi is much weaker than that of AlAl, which indicates the NiNi avoidance in La65Ni35. The major peaks of PRDFs are always smaller than the sum of efficient radius of the two kinds of atoms, especially for LaNi pairs. Atomic structure of the two systems are coincident with dense atomic packing scheme and the difference between major Voronoi polyhedron types (0, 3, 6, 0 for La65Ni35 and 0, 2, 8, 1, 0, 2, 8, 0 for La65Al35) in local structures is controlled by their ratio of solute to solvent atomic size. The high five-fold local symmetry structure gradually increases in both systems with the decrease of temperature, which validates its pivotal part in hindering crystallization. The electronic structure is studied with the partial density of states. It is found that the significant bond-shortening between La and Ni is due to the strong hybridization between Ni-3d and La-5d electrons and this result may play a key role in understanding composition related structure and properties in MGs.

Determination of the structural properties of the aqueous electrolyte LiCl6H2O at the supercooled state using the Reverse Monte Carlo (RMC) simulation

A structural study of an aqueous electrolyte whose experimental results are available. It is a solution of A structural study of an aqueous electrolyte whose experimental results are available. It is a solution LiCl6H2O type at supercooled state (162K) contrasted with pure water at room temperature by means of Partial Distribution Functions (PDF) issue from neutron scattering technique. The aqueous electrolyte solution of the chloride lithium LiCl presents interesting properties which is studied by different methods at different concentration and thermodynamical states: This system possesses the property to become a glass through a metastable supercooled state when the temperature decreases. Based on these partial functions, the Reverse Monte Carlo method (RMC) computes radial correlation functions which allow exploring a number of structural features of the system. The purpose of the RMC is to produce a consistent configuration with the experimental data. They are usually the most important in the limit of systematic errors (of unknown distribution).

Neutron Scattering of Aromatic and Aliphatic Liquids.

Organic solvents, such as cyclohexane, cyclohexene, methylcyclohexane, benzene and toluene, are widely used as both reagents and solvents in industrial processes. Despite the ubiquity of these liquids, the local structures that govern the chemical properties have not been studied extensively. Herein, we report neutron diffraction measurements on liquid cyclohexane, cyclohexene, methylcyclohexane, benzene and toluene at 298 K to obtain a detailed description of the local structure in these compounds. The radial distribution functions of the centres of the molecules, as well as the partial distribution functions for the double bond for cyclohexene and methyl group for methylcyclohexane and toluene have been calculated. Additionally, probability density functions and angular radial distribution functions were extracted to provide a full description of the local structure within the chosen liquids. Structural motifs are discussed and compared for all liquids, referring specifically to the functional group and aromaticity present in the different liquids.

Cornish–Fisher expansions for functionals of the partial sum empirical distribution

Given a random sample X1,…,Xn in Rp from some distribution function F we define the partial sum empirical distribution function as Gn(x,t)=n−1∑i=1[nt]I(Xi≤x) for x in Rp, 0≤t≤1. We give Cornish–Fisher expansions for smooth functionals of Gn. Applications to sequential analysis include cusum-type functionals for monitoring variance, and a Studentized cusum-type functional for monitoring the mean.

Structural modelling of the LiCl aqueous solution by the hybrid reverse Monte Carlo (HRMC) simulation

The reverse Monte Carlo (RMC) simulation is applied in the study of an aqueous electrolyte LiCl6H2O. On the basis of the available experimental neutron scattering data, RMC computes pair radial distribution functions in order to explore the structural features of the system. The obtained results include some unrealistic features. To overcome this problem, we use the hybrid reverse Monte Carlo (HRMC), incorporating an additional energy constraint in addition to the usual constraints of the pair correlation functions and average coordination. Our results show a good agreement between experimental and computed partial distribution functions (PDFs) as well as a significant improvement in pair partial distribution curves. This kind of study can be considered as a useful test for a defined interaction model for conventional simulation techniques

The Behavior of a TiO2 Nanoparticle under Extreme Conditions

ABSTRACTCompressibility of anatase nano particles of TiO2 changes from the bulk counterpart. This has been associated with amorphization and compaction. The behavior of such systems under extreme conditions is examined using a shell partial distribution function and some comparison made with rutile and baddeylite polytypes based nano structures. Particle energies of rutile and baddeylite nano particles appear to be rather size independent as compared to the anatase polytypes. The latter is associated with large relaxations and re-bonding in the relatively soft anatase phase of nano TiO2.

Variational Principle in the Theory of Partial Distribution Functions

We have constructed thermodynamically consistent theory of correlations in a classical equilibrium system. The theory is based on a variational principle for thermodynamic potential, as functional of correlation functions. Both the thermodynamic potential and correlation functions are determined simultaneously from a uniform variation problem for this functional. We have offered also methods of consecutive approximations system construction for the solution of the obtained variational problem.

Diffusion of Hydrogen in Amorphous Ni-Zr Alloys

Amorphous alloys Ni64Zr36 and Ni36Zr64 structure and hydrogen mobility are researched by the molecular dynamics method. The analysis of structure factors and partial distribution functions of atoms revealed hydrogen affects the short order parameters of the disordered systems. Diffusion coefficients of hydrogen are shown to depend on its concentration.